The present invention relates to an air flow meter assembly and, more particularly, to a hot-wire air flow meter assembly suitable for use in a fuel supplying device for internal combustion engines.
Various types of air flow meters for internal combustion engines have been proposed and, for example, in Japanese Patent Laid Open application No. 55-37555 (Mar. 15, 1980), an air flow meter is proposed having a hot-wire disposed in a by-pass air passage provided in parallel with an air passage leading from an air cleaner and having a venturi portion and a throttle valve through which the air is supplied to the engine. An output signal from the hot-wire corresponds to the total air flow supplied to the engine through the air passage, and the output signal is delivered, for example, to a control unit for calculation of an optimum air-fuel ratio of the mixture. A fuel injector or a carburetor is controlled in accordance with the optimum air-fuel ratio calculated in the control unit, so that the mixture is fed to the engine at the optimized air-fuel ratio.
A prior-art air flow meter of the aforementioned type has a number of disadvantages. Namely, the air flowing into the air passage through the air cleaner generates vortexes to form complicated stream lines. This turbulence of the air adversely affects the air flowing through the by-pass air passage to cause a disturbance of the output signal from the hot-wire, resulting in an incorrect indication of the air flow.
Additionally, it is often experienced that contaminants such as carbon particles, oil particles and so forth come into the by-pass passage due to the phenomenon peculiar in engines such as back-firing. These contaminants undesirably attach to the surface of the hot-wire to deteriorate the sensitivity thereof.